Breast cancer is the most common cancer afflicting women in the United States and the second leading cause of cancer-related deaths. Estrogen receptor (ER) status is used clinically both as a prognostic factor and as a guide in the therapy of breast cancer. One-half of all breast cancers express high levels of ER; of these, two-thirds will respond to therapy with antiestrogens such as tamoxifen. In addition to the treatment of advanced breast cancer, tamoxifen is effective adjuvant therapy in increasing numbers of women with ER-positive tumors. Of potentially even wider importance, clinical trials of tamoxifen as prophylactic therapy in women at increased risk for the development of breast cancer are underway. A dissection of the details of ER action at the molecular level is therefore timely and of clinical significance. The goal of this proposal is the elucidation of the mechanism by which the ER functions as a hormone-dependent transcription factor. This is a part of a long-term goal of understanding in detail the factors that underlie the control of hormonal responsiveness of human breast cancer. The central hypothesis to be addressed by this proposal is that additional specific nuclear factors are required for the hormone dependent transcriptional activation mediated by ER. These factors will either be part of the class II transcriptional machinery or accessory factors such as adaptors or co-activators. Two proteins, ERAP160 and ERAP140, potentially important in the ligand- dependent transactivation function of the ER have been identified. These proteins were found on the basis of their ability to bind the hormone binding domain of the ER in a hormone-dependent manner. Significantly a full length cDNA encoding ERAP140 has been cloned revealing it to be a novel protein. The aims of this proposal are to identify and clone the other ERAPs including ERAP160 and to analyze the function of the cloned ERAPs both in vivo and in vitro. Ultimately, an understanding of the function of these factors and their role in the pathway of ER-mediated transactivation will contribute to the development of new approaches in the prevention and treatment of breast cancer.